An aggregate of useless nuclear fuel coming to an end of burn-up at the termination of a nuclear fuel cycle is called the spent nuclear fuel. Currently, the spent nuclear fuel is stored and managed in a storage facility until it is reprocessed. For example, in a storage system of fuel pool type, a SUS rack including bundles of square pipes is sunken in a pool, and the aggregate of spent nuclear fuel is accommodated in the square pipes to satisfy the needs for cooling effect, shielding effect and non-criticality.
In recent years, a start has made at employing a boron-doped stainless material for square pipes to configure a rack. The use of such the square pipes can omit a neutron absorber material disposed between the square pipes to eliminate a gap between the square pipes. Therefore, it is possible to increase the number of square pipes that can be inserted in a pit in the pool and accordingly increase the number of the aggregates of the spent nuclear fuel to be accommodated.
The above square pipes can be applied to various storage systems including types of cask, lateral silo, pool and bold. It is required to produce a large number of square pipes to configure even a rack, and accordingly a technology capable of producing square pipes efficiently is required. It is also required to absorb neutrons radiated from the aggregate of spent nuclear fuel. Therefore, the square pipes are required to have soundness in their structures.
The square pipes are employed for storing the aggregate of spent nuclear fuel. Other than the rack of square pipe type, a flat plate type is also known, which requires efficiency in the production and soundness in the structure. The present invention relates to a method of manufacturing such square pipes, for example.